By way of brief background concerning the Media Access Control (MAC) layer, in the Open Systems Interconnection (OSI) model of communication and telecommunication protocols, the MAC layer is one of two sublayers of the Data Link Control (DLC) layer and is concerned with sharing the physical connection to the network among several computers. The other sublayer level in the DLC layer is the Logical Link Control (LLC) sublayer. Making a connection to an Ethernet network usually requires the use of MAC and Physical-Layer Interface (PHY) chips. Currently, from a hardware standpoint, the MAC connects to the PHY using a standard Medium Independent Interface (MII) bus, so different MACs and PHYs can communicate together.
In this regard, each computer has its own unique MAC address that is used by the MAC sublayer of the DLC layer. There is a different MAC sublayer for each physical device type. In a local area network (LAN) or other network, the MAC address of a computer is its unique hardware number. Ethernet is an example of a protocol that works at the MAC layer level. On an Ethernet LAN, the MAC address is the same as the Ethernet address. When connected to the Internet from a computer, or host in terms of the Internet Protocol (IP), a correspondence table relates the computer's IP address to the computer's physical MAC address on the LAN. As described below, however, there are several inflexible limitations to today's MAC layer that render it difficult to achieve easy and reliable network communications with different heterogeneous radios and devices.
Due to their low costs, ease of deployment, increased coverage, and enhanced capacity (e.g., via spatial reuse), multi-hop wireless networks such as mesh networks that utilize inexpensive and readily available 802.11 wireless interfaces are touted as the new frontier of wireless networking. In addition to traditional data services, multi-hop wireless networks have the potential to deliver exciting new real-time services, such as Voice over IP (VoIP) and/or streaming music or video, thereby providing a competitive alternative to cellular networks, and in particular, for geographical areas where cellular networks are not available.
However, there are several challenges in effectively realizing real-time services over multi-hop wireless networks. First, unlike cellular networks where the bandwidth needed for a voice call is reserved (e.g., via CDMA), in an 802.11-based wireless network, all nodes share and compete for the same media (spectrum). Hence, transmissions from neighboring nodes may interfere with each other, causing collision. Although collision can be reduced by the CSMA/CA media access control (MAC) mechanism employed by 802.11, e.g., in distributed coordination function (DCF) mode, such mechanism introduces considerable overhead in terms of air time cost in transmission: although VoIP payloads themselves consume a relatively small amount of air time, the overhead introduced by 802.11 DCF MAC packet headers, MAC acknowledge (ACK) messages, and collision avoidance can be fairly significant, consuming valuable wireless capacity. This problem is further compounded in a multi-hop wireless network where packets are relayed across multiple hops, at any one of which the packet(s) may experience interference or collision. In short, network performance degrades greatly when 802.11 DCF MAC is used in a multi-hop scenario.
Furthermore, since real-time services, such as VoIP, are likely to co-exist with data services over a multi-hop wireless network, using 802.11 DCF MAC delay-sensitive real-time traffic, such as VoIP packets, ends up competing with delay-insensitive “best-effort” (BE) data traffic for access to shared media. On the Internet and in other networks, QoS (Quality of Service) is the idea that transmission rates, error rates, and other characteristics can be measured, improved, and, to some extent, guaranteed in advance. Although enhanced DCF (eDCF), which is the QoS-enhanced 802.11 MAC mechanism, has been developed for infrastructured (i.e., with access points) wireless LANs (WLANs), which works by appropriately controlling a Contention Window (CW) and Inter-Frame Spacing (IFS), eDCF does not provide adequate service differentiation for support of real-time traffic in multi-hop wireless networks because of the hidden terminal and other interference issues.
To support delay-sensitive real-time services—for example, to support VoIP—over 802.11 -based multi-hop wireless networks, it would thus be desirable to provide coordination among nodes to regulate and control transmission of VoIP packets and BE data packets for reducing collision and meeting QoS requirements. In particular, it would be desirable to implement such coordination among nodes in a distributed manner using pre-existing standard 802.11 MAC interfaces to avoid having to customize drivers and/or modify existing hardware.